[FFmpeg-devel] [PATCH] cbs_h2645: Fix infinite loop in more_rbsp_data

Andreas Rheinhardt andreas.rheinhardt at gmail.com
Wed Jun 5 05:18:54 EEST 2019

cbs_h2645_read_more_rbsp_data does not handle malformed input very well:
1. If there were <= 8 bits left in the bitreader, these bits were read
via show_bits. But show_bits requires the number of bits to be read to
be > 0 (internally it shifts by 32 - number of bits to be read which is
undefined behaviour if said number is zero; there is also an assert for
this, but it is only an av_assert2). Furthermore, in this case a shift
by -1 was performed which is of course undefined behaviour, too.
2. If there were > 0 and <= 8 bits left and all of them were zero
(this can only happen for defective input), it was reported that there
was further RBSP data.

This can lead to an infinite loop in H.265's cbs_h265_read_extension_data
corresponding to the [vsp]ps_extension_data_flag syntax elements. If the
relevant flag indicates the (potential) occurence of these syntax elements,
while all bits after this flag are zero, cbs_h2645_read_more_rbsp_data
always returns 1 on x86. Given that a checked bitstream reader is used,
we are also not "saved" by an overflow in the bitstream reader's index.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
 libavcodec/cbs_h2645.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/libavcodec/cbs_h2645.c b/libavcodec/cbs_h2645.c
index 0456937710..becb63a290 100644
--- a/libavcodec/cbs_h2645.c
+++ b/libavcodec/cbs_h2645.c
@@ -328,9 +328,11 @@ static int cbs_h2645_read_more_rbsp_data(GetBitContext *gbc)
     int bits_left = get_bits_left(gbc);
     if (bits_left > 8)
         return 1;
-    if (show_bits(gbc, bits_left) == 1 << (bits_left - 1))
+    if (bits_left == 0)
         return 0;
-    return 1;
+    if (show_bits(gbc, bits_left) & MAX_UINT_BITS(bits_left - 1))
+        return 1;
+    return 0;
 #define more_rbsp_data(var) ((var) = cbs_h2645_read_more_rbsp_data(rw))

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